1. Field of the Invention
The invention is directed to an apparatus for measuring weak, location-dependent and time-dependent magnetic fields which emanate from a source situated in an examination subject; the invention is also directed to a method for the operation of such an apparatus.
2. Description of the Prior Art
Devices are gaining increasing significance in medical diagnostics for measuring biomagnetic signals, i.e. signals that emanate from sources situated in the body of a life form (see the periodical "Bild der Wissenschaft", No. 8, 1986, pages 76 through 83). Such devices must be able to mensurationally acquire the extremely weak biomagnetic signals, for example the magnetic fields emanating from the human brain or from the human heart, whose field strength is on the order of magnitude of 10.sup.-12 T and below. These signals are required in medical diagnostics for producing magnetoencephalograms (MEG) and magnetocardiograms (MCG), which offer the advantage over electroencephalograms (EEG) and electrocardiograms (ECG) that the distortions occurring therein, caused by the passage of the currents to be acquired through the human tissue, are noticeably lower. Of particular interest for experimental research and diagnostics are the magnetic fields produced in the brain of a life form provided as examination subject which, for example, arise due to acoustic and optical stimulation of the senses of the examination subject.
Devices of the species initially cited usually have a bearing means for the acceptance of the examination subject, a sensor arrangement for measuring magnetic fields, a holder for the sensor arrangement, means for adjusting the bearing means and the sensor arrangement relative to one another, and an electronic means for amplifying and evaluating the signals deriving from the sensor arrangement, the electronic means comprises a data processing system for evaluating the acquired signals and an input means for measured results. A chamber that surrounds the bearing means and the sensor arrangement can be provided for shielding magnetic fields (shielded room). As a rule, the sensor arrangement comprises one or more gradiometers (field measuring coils with allocated compensation coils) of the first or of a higher order, a plurality of SQUIDs (superconducting quantum interference devices) corresponding in number to the plurality of gradiometers, whereby every gradiometer is inductively coupled to one of the SQUIDs. The sensor arrangement also includes a cryostat, wherein a temperature at which the SQUIDs and the gradiometers are superconductive prevails. As a rule, the cryostat is filled with liquid helium, i.e. a temperature of 4.2.degree. K. is present in the interior thereof.
It is theoretically possible with apparatus of this species to identify the local field strength of the magnetic field emanating from a source dependent on time. It is also possible, by suitable evaluation of the signals derived from the sensor arrangement upon consideration of the position of the sensor arrangement relative to the examination subject, to identify, for example, the spatial position of a source of a magnetic field situated in the inside of the examination subject. This is done on the basis of suitable calculating methods that sequence at the data processing system to which the signals of the sensor arrangement are supplied.
Essentially two problems arise. In view of the low field strength of the magnetic fields to be measured, first, the sensor arrangement must supply high-quality signals that are free of disturbances produced by environmental influences, for example noise fields, radio frequency fields or mechanical vibrations. Second, the signals derived from the sensor arrangement must be processed in the data processing system such that an sufficiently exact coincidence of the results acquired with the data processing system and the actual conditions is present. There is a relationship between the two problems insofar as only the solution of both problems can lead to results that coincide with reality with sufficient precision.
The efforts previously undertaken to resolve the former problem were directed at least to the sensor arrangement and to the shielded room.
For example, sensor arrangements were developed that contain extremely well-balanced gradiometers of the second order, that are balanced to 10.sup.-4 or better, i.e. their sensitivity is reduced by the factor 10.sup.4 or more for uniform fields ("Design and Performance of a 14-Channel Neuromagnetometer", Crum et al, 1985). The sensor arrangement disclosed therein contains a total of fourteen gradiometers of the second order arranged into cryostats, in groups of seven gradiometers each. The manufacture and operation of such a well-balanced sensor arrangement cause considerable outlay and costs. Even the manufacture of individual gradiometers of the second order having the aforementioned low sensitivity for uniform fields involves a considerable outlay. Additional measures for reducing interactive electrical and mechanical disturbances between the individual gradiometers and between the gradiometers and their mount must be undertaken in the arrangement and operation in an array of gradiometers of the second order. The outlay for the measuring instrument described in the publication is thereby further increased in that measures for measuring magnetic noise fields and for electronic compensation of the measured noise fields are undertaken.
Further, a shielded room executed in sevenshell structure having extremely high shielding effect for magnetic fields has been developed whose inner shell is formed of copper, whereas the remaining shells are composed of mu-metal (A. Mayer, "The Berlin Magnetically Shielded Room (BMSR)", Biomagnetism, Walter de Gruyter & Co., Berlin-New York, 1981, pages 73 ff). This structure leads, first, to an extremely high shielding effect of 3.times.10.sup.4 at 0.4 Hz, 3.times.10.sup.5 at 5 Hz, 1.5.times.10.sup.5 at 50 Hz and 10.sup.6 at higher frequencies, and thereby enables the employment the uncompensated measuring coils. This is achieved, however, on the basis of extremely high outlay for the structure and corresponding costs.
A further shielding room having extremely high shielding effect is described in V. Kelha, "Construction and Performance of the Otaniemi Magnetically Shielded Room", in Biomagnetism, 1981, pages 33 through 50. Comparatively high shielding factors are achieved with this shielded room that includes three mu-metal shells, each of which is enclosed between two aluminum layers. Controlled, active shielding is also provided as well as further shielding measures on the basis of what is referred to as the shaking method. Here, too, however, the outlay for constructing a total of nine shells and for the operation of the room is considerable. In addition, an effect that deteriorates the quality of the signals results because the innermost layer of the shielded room is composed of aluminum and, thus, disturbing influences due to eddy currents in the aluminum shell appear.
The sensor arrangements and shielded rooms of the prior art, accordingly, do not allow an apparatus of the species initially cited to be realized that is able to supply measured signals with the required quality.
The fundamental aspects of an operating method for apparatus of the species initially cited are disclosed in U.S. Pat. No. 4,736,751. This method assumes a sensor arrangement of at least thirty-two gradiometers; however, no details are provided regarding what quality the signals of gradiometers must have for the known method and with what apparatus signals can be acquired having adequate quality.